1. Field of the Invention
The present invention relates to a method for demodulating the signals of auxiliary channels and an FM microwave radio frequency repeater using the straight-through system, with a plurality of channels having different carrier frequencies, each said channel including an auxiliary channel.
2. Description of the Prior Art
With the progress in microwave semiconductor technology, a type of multichannel microwave radio frequency repeater having a simple construction has come into practical use. This repeater is of the type that comprises receiving frequency-modulated radio-frequency signals of the microwave frequency band and transmitting the signals thus received after direct amplification thereof without converting them into intermediate frequencies, or transmitting said received signals after slightly shifting the frequencies thereof for prevention of any interference due to the coupling of the antennas.
In the above-mentioned type of microwave radio frequency repeater, almost all of its transmission characteristics depend upon the characteristics of the amplifier provided therein. At the present time good characteristics are available with such type of microwave radio frequency repeater by virtue of the microwave semiconductors used therein. However, a repeater station equipped with such type of microwave repeater still has a problem with respect to the demodulation of signals for the auxiliary channels. Usually the repeater station is not attended by any operator but is periodically visited by maintenance personnel for inspection. This situation necessitates provision of an auxiliary channel in the microwave repeater, for emergencies, such as trouble with or an accident at the repeater station, for giving an alarm to the terminal station, or allowing various controls from the terminal station to the unattended repeater station or communication from the maintenance personnel to the terminal station for necessary arrangements, etc. Such auxiliary channel can find its effective use in a conventional type, i.e., non-straight-through type, of repeater station since, in such station, received signals can be demodulated by a demodulating circuit after their conversion into intermediate frequencies. However, in the case of a microwave radio frequency repeater using the straight-through system, the repeater is no longer one of a simple construction if equipped with an independent microwave local oscillator for conversion of the picked-up signals into intermediate frequencies. Further, no satisfactory high-sensitive microwave frequency discriminator is currently available, although such a discriminator is necessary for demodulating an auxiliary channel signal through direct FM detection of the microwave. Thus, with today's art the auxiliary channel is difficult to employ in a repeater station equipped with such a microwave repeater using the straight-through system.